Topology and Hybrid Modulation of Six-Level Switched-Capacitor HC Converter With Improved DC-Link Capacitor Voltage Balancing

Neutral point (NP) voltage imbalance blocks the deployment of carrier-based clamped converters for specific scenarios due to the limited adjustment capability of NP current by zero-sequence voltage (ZSV) injection. Therefore, a novel six-level switched-capacitor (SC) hybrid clamped (6L-SCHC) converter is proposed, which has compromised voltage stress, and utilizes the discrete voltage boost and capacitor voltage self-balancing features of the SC circuit to simplify control complexity. On this basis, a carrier-based hybrid modulation is developed, which simplifies the complexity of SC voltage control, and ensures the voltage natural balance of SC and dc-link center capacitor to minimize voltage ripples. Furthermore, a reasonable functional relationship between average NP current and reference modulation voltage is explored. As the result, the proposed 6L-SCHC converter has extremely powerful NP voltage balancing capability compared with other carrier-based clamped converters in theory, especially at high modulation indexes and low power factors. Capacitor voltage ripple models are also derived to achieve voltage independent controls of all capacitors. Comparative simulations and experiments demonstrate that the proposed topology and modulation greatly reduce the low-frequency voltage ripples of dc-link upper and lower capacitors, and provide excellent NP voltage balancing performance over a wide range of modulation indexes and power factors.

I. Introduction

The clamped multilevel converters employ devices clamped to neutral point (NP) to obtain additional switching states and reduce the usage of floating capacitors. Therefore, the clamped converters perform remarkably in medium and high voltage applications due to their high power density, low dv/dt, and appropriate control complexity [1], [2], [3], [4]. The neutral point clamped (NPC) converter, one of the most classic clamped multilevel topologies, has been widely commercialized [5], [6], [7]. However, the NPC converter suffers from serious NP voltage imbalance problems when it is extended to four-level or more, which blocks its deployment in higher voltage applications.